This invention relates to crossed-field amplifiers and more particularly to cathode-driven crossed-field amplifiers.
In the cathode-driven crossed-field the cathode is formed to provide a periodic structure which acts as a source of an emitter of electrons and as a support for RF drive energy traveling at a prescribed angular velocity around the cathode structure. The RF drive energy is introduced into the cathode slow wave structure by a transition which allows RF energy to be introduced into the cathode at ground potential while allowing the cathodes to be at a high direct current voltage with respect to ground.
Referring now to FIG. 1, a cross-sectional view of the DC block of the prior art which provided a transition from Rf energy ground potential to the RF energy on the CFA cathode at high negative potential. The transition shown in FIG. 1 is a coaxial line tranmission wherein the coaxial line portion 10 is shown as terminated prior to being connected to the CFA cathode slow wave structure. The transmission line section 10 is composed of an outer conductor 11, an inner conductor 12 and a window 13 typically a ceramic window. The window 13 is hermetically sealed to the outer and inner conductors 11, 12 to allow the region 14 which communicates with the interior of the CFA tube to be in a vacuum whereas the region 15 on the other side of the seal 13 is exposed to the atmosphere. A glass member 16 electrically isolates the coaxial line 10 which is at the high negative cathode potential from the transmission line 17 which desirably is at ground potential. Glass member 16 is shaped to accommodate the inner and outer conductors of both coaxial lines 10 and 17. The inner most portion of glass member 16 has a metallic lining 19 which is in electrical contact with the center conductor 12 having a bulbless springed end 20, the inner conductor 21 of coaxial line 17 has a slotted metallic extension 22 which slips over the inner most portion 23 of glass member 16. Metallic section 22 overlaps the metallic coating 19 by a quarter wavelenght of the designed frequency for the transition. The outer conductors 11 and 18 overlap in a similar manner in order to provide a virtual short between the conductors to cause the two transmission lines 10 and 17 to be effectively coupled with respect to alternating frequency but to be direct current isolated by the glass member 16. The remainder of transmission line 17 is a conventional coaxial line with a stub support 24 and a ceramic 25 centering piece for the inner conductor 21. Coupling to an external transmission line 26 is made through coupler 27 in a conventional manner.
The prior art DC block illustrated in FIG. 1 has the disadvantage that an arc over of the high DC voltage which exists on the cathode either from the outer conductor or the inner conductor of the coaxial line 10 to the corresponding conductors of the coaxial line 11 will cause coaxial line 17 to be at an elevated voltage even though coaxial conductor may be grounded as indicated by ground connection 28 because of the resistance to ground and the large current capacity of the power supply that provides the high negative voltage to the crossed-field amplifier cathode. Even more serious is the possibility of an arc over when through inadvertents or otherwise there is no ground connection 28 at which an operator making a connection of the transmission line 26 to the crossed-field amplifier transmission line 17 would be subjected to the high voltage of the cathode. Thus, the prior art DC block subjects the operator to the possibility of lethal voltage potentials because of arc over across the insulating material 16. Because of the construction of the crossed-field amplifier the insulator material 16 is exposed to the atmosphere and is not under vacuum and hence arc over may be precipitated by atmospheric moisture or contaminants surrounding or deposited on the insulator 16.
It is therefore a primary object of this invention to provide a DC block which does not have the disadvantages of the prior art block. More specifically, the DC block of this invention has as an object the providing of DC voltage isolation independent of the atmospheric conditions attendant upon the operation of the tube. It is a further object of this invention to provide a DC block which is safer from an operators standpoint. It is a still further object of the invention to provide a DC block which is more rugged than the prior art DC block and has more tolerance to shock or mishandling of the tube.
It is a feature of this invention that these and other objects are achieved in this invention by providing a structure in which the direct current isolating coupler is contained within a vacuum thereby rendering it immune to atmospheric conditions which may degrade its performance.